1. Field of the Invention
The invention relates to a measuring apparatus of an air flow rate and, more particularly, to a hot-wire type air flow meter (heat flow meter) suitable for detection of a suction air flow rate of an internal combustion engine.
2. Description of Related Art
Hitherto, as an air flow rate measuring apparatus which is provided for an electronic control type fuel injection apparatus of an internal combustion engine such as an automobile or the like and measures a suction air flow rate, a number of hot-wire type apparatuses have been used because a mass air flow rate can be directly detected. In this instance, to assure a reliability of an exothermic resistor, a sensor having a certain extent of heat capacity which is constructed by winding a platinum wire around a bobbin and coating it with glass, forming a thin film resistor onto a ceramic substrate, or the like is often used. In a sensor having a large heat capacity, although a system such that a current is limited when a waterdrop is deposited onto a portion other than an internal combustion engine has been shown in JP-A-5-264312, a power voltage is large to be 24V and a measuring region of a rated value or more is limited. Such a system is used, particularly, for measurement of a flow rate of the compressed air and there is a problem that although a waterdrop is instantaneously evaporated usually at 100xc2x0 C. at an atmospheric pressure, in the compressed air, an evaporation temperature of the water is high, it takes time to evaporate the waterdrop, and an excessive current flows in an exothermic portion. In this instance, since the power voltage is high to be 24V, when a waterdrop is deposited, a current which is one or more digits larger than a rated current flows, and accordingly, it is limited by a current limiter. In an internal combustion engine, to prevent a fusion of a hot wire, a system for reducing an applied voltage to a bridge by detecting a voltage across a hot wire resistor has been disclosed in JP-A-61-102522 or the like. According to such a system, to prevent the fusion due to the supply of the abnormal voltage because of noises or the like, a voltage limit value larger than the maximum value at the time of the stationary operation is set.
As a flow meter having a high response speed, a number of flow meters in which an exothermic resistor of a small heat capacity is attached on a silicon substrate have been proposed. For example, as shown in JP-B-5-7659, a thin film is formed on a silicon semiconductor substrate and a flow rate is detected by a thermal sensor.
In the conventional techniques mentioned above, in case of using an exothermic resistor of a large heat capacity, a response speed at the time of a power-on of an air flow rate measuring apparatus is determined only from the relation between a power source voltage and the heat capacity of the exothermic resistor. In an internal combustion engine such as an ordinary gasoline engine or the like, therefore, since a power source voltage is usually small to be about 12V, an operating current to the exothermic resistor is limited and it takes time to heat, so that it is difficult to accurately measure an air flow rate for a predetermined time at the start of an engine of an automobile or the like.
To improve the response speed, it is necessary to use an exothermic resistor of a small heat capacity. In a hot-wire air flow meter, however, the maximum flow rate which can be measured is limited unconditionally to a certain degree by the heat capacity and a heating temperature of the exothermic resistor which is used for measurement and by a power source voltage which can be applied. It is because when the heat capacity is small, if it is intended to raise a measuring sensitivity (maximum flow rate) of the flow rate by increasing the heating temperature, a limitation such that the exothermic resistor deteriorates when the heating temperature is equal to or higher than a predetermined value occurs. For example, a system such that an exothermic resistor is constructed by forming a thin film with a diaphragm structure onto a silicon substrate and a temperature of the exothermic resistor is controlled so as to become a constant temperature by a feedback circuit by using a bridge circuit or the like has the highest response speed. If a waterdrop, an oil droplet, or the like is deposited onto the exothermic resistor portion, however, a heating balance is broken and a voltage is applied so that the heating temperature of the exothermic resistor locally rises. Even in case of the applied voltage which does not cause a problem in the ordinary operation, therefore, if the temperature elevation continues, there is a possibility that the resistor of a small heat capacity constructed by the thin film is deteriorated by the heat and, in the worst case, the diaphragm is broken by a thermal stress.
Although the excessive temperature elevation can be prevented by limiting the voltage applied to the hot-wire type resistor so as not to locally raise the heating temperature of the exothermic resistor, there is a problem that a measuring range of the inherent flow rate is also narrowed.
It is, therefore, an object of the invention to provide a hot-wire type air flow meter for an internal combustion engine which can solve the problems of the conventional techniques mentioned above.
According to an aspect of the invention, there are provided: means for detecting a heating state of an exothermic resistor by distinguishing a case where a waterdrop, an oil droplet, or the like is deposited onto the exothermic resistor portion from a normal case where there is no deposition; and circuit means for limiting a heating time by the detected heating state. Thus, only in the case where the waterdrop, oil droplet, or the like is deposited onto the exothermic resistor portion, the heating state is limited without narrowing a measuring range of an inherent flow rate and a deterioration of the resistor can be prevented.